An erythromycin-resistance plasmid, pAMbeta1, from a strain of Streptococcus faecalis was shown to mediate its own transfer, by a mechanism resembling conjugation, among several species of streptococci including certain strains of S. mutans, S. sanguis, and S. salivarius. Evidence has also been obtained which suggests that this 17 megadalton plasmid is a composite molecule consisting of two independent replicons with molecular weights of 10 times 10 to the 6th power and 7 times 10 to the 6th power. Studies on patients treated for periodontal disease with tetracycline (Tc) suggested that this antibiotic selects for a highly resistant oral microflora. The Tc-resistant trait has been shown to be plasmid-mediated in some of these isolates, and one strain of S. sanguis transferred the resistance to a strain of S. mutans during cell-to-cell contact. The ability of Lancefield group N streptococci to metabolize lactose is known to be governed by plasmids. The lactose plasmid of one strain of S. lactis was also shown to influence the biochemical pathway by which galactose is utilized by this organism.